Model the tied end of a nylon/plastic bag
Modeling

Go to a new scene and delete the default cube. Shift + Add a Cylinder mesh. Tab into Edit Mode. Key Numpad 1 and then Numpad 5 to enter Front Orthographic View. Key Z to enter Wireframe View and Box deselect the top verticies (drag to create the box with the middle-mouse button). Key S then .2 to scale the bottom vertices. Your mesh should presently look like this:



Change into Edge Select Mode, key A once or twice to deselect everything, and Box select all lateral edges (the ones along the side going from top to bottom). At the bottom of the 3D Viewport, go to Select > Checker Deselect. Your mesh should have these edges selected:



Key S, then Shift + Z, then .9. Enter Face Select Mode. Select the top and bottom faces (bases) of the mesh. Key X and select Faces. Your mesh should now look like this:



Reenter Vertex Select Mode. Select two vertices opposite each other on the larger side of the mesh. Key O to turn on Proportional editing. Set the Falloff type to Random. Key S to Scale and move the mouse's scroll wheel one way or another until the white circle covers the whole mesh. Scale inwards.



Tab into Object Mode. Go to the Object modifiers tab in the Properties panel. Add a Subdivision Surface modifier. Set both the View: and Render: values to 2. In the 3D Viewport's Tools panel, select Shading: Smooth.

Texturing

Go to the Materials tab in the Properties panel. + New to add a new material. Select Add Shader from the shader drop down menu. In the top slot under the Add Shader, select Translucent BSDF. In the bottom slot under the Add Shader, select Refraction BSDF. If you go into the Node Editor, you may find something like this:



Finished .blend file: